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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-MATCHING"
>9.7. Pattern Matching</A
></H1
><P
>    There are three separate approaches to pattern matching provided
    by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>: the traditional
    <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> <CODE
CLASS="FUNCTION"
>LIKE</CODE
> operator, the
    more recent <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> operator (added in
    SQL:1999), and <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
>-style regular
    expressions.  Aside from the basic <SPAN
CLASS="QUOTE"
>"does this string match
    this pattern?"</SPAN
> operators, functions are available to extract
    or replace matching substrings and to split a string at matching
    locations.
   </P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
>     If you have pattern matching needs that go beyond this,
     consider writing a user-defined function in Perl or Tcl.
    </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-LIKE"
>9.7.1. <CODE
CLASS="FUNCTION"
>LIKE</CODE
></A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> LIKE <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>ESCAPE <TT
CLASS="REPLACEABLE"
><I
>escape-character</I
></TT
></SPAN
>]
<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> NOT LIKE <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>ESCAPE <TT
CLASS="REPLACEABLE"
><I
>escape-character</I
></TT
></SPAN
>]</PRE
><P
>     The <CODE
CLASS="FUNCTION"
>LIKE</CODE
> expression returns true if the
     <TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> matches the supplied
     <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>.  (As
     expected, the <CODE
CLASS="FUNCTION"
>NOT LIKE</CODE
> expression returns
     false if <CODE
CLASS="FUNCTION"
>LIKE</CODE
> returns true, and vice versa.
     An equivalent expression is
     <TT
CLASS="LITERAL"
>NOT (<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> LIKE
      <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>)</TT
>.)
    </P
><P
>     If <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> does not contain percent
     signs or underscores, then the pattern only represents the string
     itself; in that case <CODE
CLASS="FUNCTION"
>LIKE</CODE
> acts like the
     equals operator.  An underscore (<TT
CLASS="LITERAL"
>_</TT
>) in
     <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> stands for (matches) any single
     character; a percent sign (<TT
CLASS="LITERAL"
>%</TT
>) matches any sequence
     of zero or more characters.
    </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>'abc' LIKE 'abc'    <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' LIKE 'a%'     <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' LIKE '_b_'    <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' LIKE 'c'      <I
CLASS="LINEANNOTATION"
>false</I
></PRE
><P>
   </P
><P
>    <CODE
CLASS="FUNCTION"
>LIKE</CODE
> pattern matching always covers the entire
    string.  Therefore, to match a sequence anywhere within a string, the
    pattern must start and end with a percent sign.
   </P
><P
>    To match a literal underscore or percent sign without matching
    other characters, the respective character in
    <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> must be
    preceded by the escape character.  The default escape
    character is the backslash but a different one can be selected by
    using the <TT
CLASS="LITERAL"
>ESCAPE</TT
> clause.  To match the escape
    character itself, write two escape characters.
   </P
><P
>    Note that the backslash already has a special meaning in string literals,
    so to write a pattern constant that contains a backslash you must write two
    backslashes in an SQL statement (assuming escape string syntax is used, see
    <A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS"
>Section 4.1.2.1</A
>).  Thus, writing a pattern that
    actually matches a literal backslash means writing four backslashes in the
    statement.  You can avoid this by selecting a different escape character
    with <TT
CLASS="LITERAL"
>ESCAPE</TT
>; then a backslash is not special to
    <CODE
CLASS="FUNCTION"
>LIKE</CODE
> anymore. (But backslash is still special to the
    string literal parser, so you still need two of them to match a backslash.)
   </P
><P
>    It's also possible to select no escape character by writing
    <TT
CLASS="LITERAL"
>ESCAPE ''</TT
>.  This effectively disables the
    escape mechanism, which makes it impossible to turn off the
    special meaning of underscore and percent signs in the pattern.
   </P
><P
>    The key word <TT
CLASS="TOKEN"
>ILIKE</TT
> can be used instead of
    <TT
CLASS="TOKEN"
>LIKE</TT
> to make the match case-insensitive according
    to the active locale.  This is not in the <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> standard but is a
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extension.
   </P
><P
>    The operator <TT
CLASS="LITERAL"
>~~</TT
> is equivalent to
    <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, and <TT
CLASS="LITERAL"
>~~*</TT
> corresponds to
    <CODE
CLASS="FUNCTION"
>ILIKE</CODE
>.  There are also
    <TT
CLASS="LITERAL"
>!~~</TT
> and <TT
CLASS="LITERAL"
>!~~*</TT
> operators that
    represent <CODE
CLASS="FUNCTION"
>NOT LIKE</CODE
> and <CODE
CLASS="FUNCTION"
>NOT
    ILIKE</CODE
>, respectively.  All of these operators are
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>-specific.
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-SIMILARTO-REGEXP"
>9.7.2. <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> Regular Expressions</A
></H2
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> SIMILAR TO <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>ESCAPE <TT
CLASS="REPLACEABLE"
><I
>escape-character</I
></TT
></SPAN
>]
<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> NOT SIMILAR TO <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>ESCAPE <TT
CLASS="REPLACEABLE"
><I
>escape-character</I
></TT
></SPAN
>]</PRE
><P
>    The <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> operator returns true or
    false depending on whether its pattern matches the given string.
    It is similar to <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, except that it
    interprets the pattern using the SQL standard's definition of a
    regular expression.  SQL regular expressions are a curious cross
    between <CODE
CLASS="FUNCTION"
>LIKE</CODE
> notation and common regular
    expression notation.
   </P
><P
>    Like <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, the <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
>
    operator succeeds only if its pattern matches the entire string;
    this is unlike common regular expression behavior where the pattern
    can match any part of the string.
    Also like
    <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> uses
    <TT
CLASS="LITERAL"
>_</TT
> and <TT
CLASS="LITERAL"
>%</TT
> as wildcard characters denoting
    any single character and any string, respectively (these are
    comparable to <TT
CLASS="LITERAL"
>.</TT
> and <TT
CLASS="LITERAL"
>.*</TT
> in POSIX regular
    expressions).
   </P
><P
>    In addition to these facilities borrowed from <CODE
CLASS="FUNCTION"
>LIKE</CODE
>,
    <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> supports these pattern-matching
    metacharacters borrowed from POSIX regular expressions:

   <P
></P
></P><UL
><LI
><P
>      <TT
CLASS="LITERAL"
>|</TT
> denotes alternation (either of two alternatives).
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>*</TT
> denotes repetition of the previous item zero
      or more times.
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>+</TT
> denotes repetition of the previous item one
      or more times.
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>?</TT
> denotes repetition of the previous item zero
      or one time.
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}</TT
> denotes repetition
      of the previous item exactly <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> times.
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,}</TT
> denotes repetition
      of the previous item <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> or more times.
     </P
></LI
><LI
><P
>      <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}</TT
>
      denotes repetition of the previous item at least <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> and
      not more than <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> times.
     </P
></LI
><LI
><P
>      Parentheses <TT
CLASS="LITERAL"
>()</TT
> can be used to group items into
      a single logical item.
     </P
></LI
><LI
><P
>      A bracket expression <TT
CLASS="LITERAL"
>[...]</TT
> specifies a character
      class, just as in POSIX regular expressions.
     </P
></LI
></UL
><P>

    Notice that the period (<TT
CLASS="LITERAL"
>.</TT
>) is not a metacharacter
    for <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
>.
   </P
><P
>    As with <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, a backslash disables the special meaning
    of any of these metacharacters; or a different escape character can
    be specified with <TT
CLASS="LITERAL"
>ESCAPE</TT
>.
   </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>'abc' SIMILAR TO 'abc'      <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' SIMILAR TO 'a'        <I
CLASS="LINEANNOTATION"
>false</I
>
'abc' SIMILAR TO '%(b|d)%'  <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' SIMILAR TO '(b|c)%'   <I
CLASS="LINEANNOTATION"
>false</I
></PRE
><P>
   </P
><P
>    The <CODE
CLASS="FUNCTION"
>substring</CODE
> function with three parameters,
    <CODE
CLASS="FUNCTION"
>substring(<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> from
    <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
> for
    <TT
CLASS="REPLACEABLE"
><I
>escape-character</I
></TT
>)</CODE
>, provides
    extraction of a substring that matches an SQL
    regular expression pattern.  As with <TT
CLASS="LITERAL"
>SIMILAR TO</TT
>, the
    specified pattern must match the entire data string, or else the
    function fails and returns null.  To indicate the part of the
    pattern that should be returned on success, the pattern must contain
    two occurrences of the escape character followed by a double quote
    (<TT
CLASS="LITERAL"
>"</TT
>). 
    The text matching the portion of the pattern
    between these markers is returned.
   </P
><P
>    Some examples, with <TT
CLASS="LITERAL"
>#&quot;</TT
> delimiting the return string:
</P><PRE
CLASS="PROGRAMLISTING"
>substring('foobar' from '%#"o_b#"%' for '#')   <I
CLASS="LINEANNOTATION"
>oob</I
>
substring('foobar' from '#"o_b#"%' for '#')    <I
CLASS="LINEANNOTATION"
>NULL</I
></PRE
><P>
   </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="FUNCTIONS-POSIX-REGEXP"
>9.7.3. <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> Regular Expressions</A
></H2
><P
>    <A
HREF="functions-matching.html#FUNCTIONS-POSIX-TABLE"
>Table 9-11</A
> lists the available
    operators for pattern matching using POSIX regular expressions.
   </P
><DIV
CLASS="TABLE"
><A
NAME="FUNCTIONS-POSIX-TABLE"
></A
><P
><B
>Table 9-11. Regular Expression Match Operators</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Operator</TH
><TH
>Description</TH
><TH
>Example</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>~</TT
> </TD
><TD
>Matches regular expression, case sensitive</TD
><TD
><TT
CLASS="LITERAL"
>'thomas' ~ '.*thomas.*'</TT
></TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>~*</TT
> </TD
><TD
>Matches regular expression, case insensitive</TD
><TD
><TT
CLASS="LITERAL"
>'thomas' ~* '.*Thomas.*'</TT
></TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>!~</TT
> </TD
><TD
>Does not match regular expression, case sensitive</TD
><TD
><TT
CLASS="LITERAL"
>'thomas' !~ '.*Thomas.*'</TT
></TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>!~*</TT
> </TD
><TD
>Does not match regular expression, case insensitive</TD
><TD
><TT
CLASS="LITERAL"
>'thomas' !~* '.*vadim.*'</TT
></TD
></TR
></TBODY
></TABLE
></DIV
><P
>     <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> regular expressions provide a more
     powerful means for pattern matching than the <CODE
CLASS="FUNCTION"
>LIKE</CODE
> and
     <CODE
CLASS="FUNCTION"
>SIMILAR TO</CODE
> operators.
     Many Unix tools such as <TT
CLASS="COMMAND"
>egrep</TT
>,
     <TT
CLASS="COMMAND"
>sed</TT
>, or <TT
CLASS="COMMAND"
>awk</TT
> use a pattern
     matching language that is similar to the one described here.
    </P
><P
>     A regular expression is a character sequence that is an
     abbreviated definition of a set of strings (a <I
CLASS="FIRSTTERM"
>regular
     set</I
>).  A string is said to match a regular expression
     if it is a member of the regular set described by the regular
     expression.  As with <CODE
CLASS="FUNCTION"
>LIKE</CODE
>, pattern characters
     match string characters exactly unless they are special characters
     in the regular expression language &mdash; but regular expressions use
     different special characters than <CODE
CLASS="FUNCTION"
>LIKE</CODE
> does.
     Unlike <CODE
CLASS="FUNCTION"
>LIKE</CODE
> patterns, a
     regular expression is allowed to match anywhere within a string, unless
     the regular expression is explicitly anchored to the beginning or
     end of the string.
    </P
><P
>     Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>'abc' ~ 'abc'    <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' ~ '^a'     <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' ~ '(b|d)'  <I
CLASS="LINEANNOTATION"
>true</I
>
'abc' ~ '^(b|c)' <I
CLASS="LINEANNOTATION"
>false</I
></PRE
><P>
    </P
><P
>     The <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> pattern language is described in much
     greater detail below.
    </P
><P
>     The <CODE
CLASS="FUNCTION"
>substring</CODE
> function with two parameters,
     <CODE
CLASS="FUNCTION"
>substring(<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
> from
     <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>)</CODE
>, provides extraction of a
     substring
     that matches a POSIX regular expression pattern.  It returns null if
     there is no match, otherwise the portion of the text that matched the
     pattern.  But if the pattern contains any parentheses, the portion
     of the text that matched the first parenthesized subexpression (the
     one whose left parenthesis comes first) is
     returned.  You can put parentheses around the whole expression
     if you want to use parentheses within it without triggering this
     exception.  If you need parentheses in the pattern before the
     subexpression you want to extract, see the non-capturing parentheses
     described below.
    </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>substring('foobar' from 'o.b')     <I
CLASS="LINEANNOTATION"
>oob</I
>
substring('foobar' from 'o(.)b')   <I
CLASS="LINEANNOTATION"
>o</I
></PRE
><P>
   </P
><P
>     The <CODE
CLASS="FUNCTION"
>regexp_replace</CODE
> function provides substitution of
     new text for substrings that match POSIX regular expression patterns.
     It has the syntax
     <CODE
CLASS="FUNCTION"
>regexp_replace</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>source</I
></TT
>,
     <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>replacement</I
></TT
>
     [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> </SPAN
>]).
     The <TT
CLASS="REPLACEABLE"
><I
>source</I
></TT
> string is returned unchanged if
     there is no match to the <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>.  If there is a
     match, the <TT
CLASS="REPLACEABLE"
><I
>source</I
></TT
> string is returned with the
     <TT
CLASS="REPLACEABLE"
><I
>replacement</I
></TT
> string substituted for the matching
     substring.  The <TT
CLASS="REPLACEABLE"
><I
>replacement</I
></TT
> string can contain
     <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>, where <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> is 1
     through 9, to indicate that the source substring matching the
     <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>'th parenthesized subexpression of the pattern should be
     inserted, and it can contain <TT
CLASS="LITERAL"
>\&amp;</TT
> to indicate that the
     substring matching the entire pattern should be inserted.  Write
     <TT
CLASS="LITERAL"
>\\</TT
> if you need to put a literal backslash in the replacement
     text.  (As always, remember to double backslashes written in literal
     constant strings, assuming escape string syntax is used.)
     The <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag <TT
CLASS="LITERAL"
>i</TT
> specifies case-insensitive
     matching, while flag <TT
CLASS="LITERAL"
>g</TT
> specifies replacement of each matching
     substring rather than only the first one.  Other supported flags are
     described in <A
HREF="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE"
>Table 9-19</A
>.
    </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>regexp_replace('foobarbaz', 'b..', 'X')
                                   <I
CLASS="LINEANNOTATION"
>fooXbaz</I
>
regexp_replace('foobarbaz', 'b..', 'X', 'g')
                                   <I
CLASS="LINEANNOTATION"
>fooXX</I
>
regexp_replace('foobarbaz', 'b(..)', E'X\\1Y', 'g')
                                   <I
CLASS="LINEANNOTATION"
>fooXarYXazY</I
></PRE
><P>
   </P
><P
>     The <CODE
CLASS="FUNCTION"
>regexp_matches</CODE
> function returns a text array of
     all of the captured substrings resulting from matching a POSIX
     regular expression pattern.  It has the syntax
     <CODE
CLASS="FUNCTION"
>regexp_matches</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>
     [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> </SPAN
>]).
     The function can return no rows, one row, or multiple rows (see
     the <TT
CLASS="LITERAL"
>g</TT
> flag below).  If the <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>
     does not match, the function returns no rows.  If the pattern
     contains no parenthesized subexpressions, then each row
     returned is a single-element text array containing the substring
     matching the whole pattern.  If the pattern contains parenthesized
     subexpressions, the function returns a text array whose
     <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>'th element is the substring matching the
     <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>'th parenthesized subexpression of the pattern
     (not counting <SPAN
CLASS="QUOTE"
>"non-capturing"</SPAN
> parentheses; see below for
     details).
     The <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> parameter is an optional text
     string containing zero or more single-letter flags that change the
     function's behavior.  Flag <TT
CLASS="LITERAL"
>g</TT
> causes the function to find
     each match in the string, not only the first one, and return a row for
     each such match.  Other supported
     flags are described in <A
HREF="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE"
>Table 9-19</A
>.
    </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT regexp_matches('foobarbequebaz', '(bar)(beque)');
 regexp_matches 
----------------
 {bar,beque}
(1 row)

SELECT regexp_matches('foobarbequebazilbarfbonk', '(b[^b]+)(b[^b]+)', 'g');
 regexp_matches 
----------------
 {bar,beque}
 {bazil,barf}
(2 rows)

SELECT regexp_matches('foobarbequebaz', 'barbeque');
 regexp_matches 
----------------
 {barbeque}
(1 row)</PRE
><P>
   </P
><P
>    It is possible to force <CODE
CLASS="FUNCTION"
>regexp_matches()</CODE
> to always
    return one row by using a sub-select;  this is particularly useful
    in a <TT
CLASS="LITERAL"
>SELECT</TT
> target list when you want all rows
    returned, even non-matching ones:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT col1, (SELECT regexp_matches(col2, '(bar)(beque)')) FROM tab;</PRE
><P>
   </P
><P
>     The <CODE
CLASS="FUNCTION"
>regexp_split_to_table</CODE
> function splits a string using a POSIX
     regular expression pattern as a delimiter.  It has the syntax
     <CODE
CLASS="FUNCTION"
>regexp_split_to_table</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>
     [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> </SPAN
>]).
     If there is no match to the <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>, the function returns the
     <TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
>.  If there is at least one match, for each match it returns
     the text from the end of the last match (or the beginning of the string)
     to the beginning of the match.  When there are no more matches, it
     returns the text from the end of the last match to the end of the string.
     The <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> parameter is an optional text string containing
     zero or more single-letter flags that change the function's behavior.
     <CODE
CLASS="FUNCTION"
>regexp_split_to_table</CODE
> supports the flags described in
     <A
HREF="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE"
>Table 9-19</A
>.
    </P
><P
>     The <CODE
CLASS="FUNCTION"
>regexp_split_to_array</CODE
> function behaves the same as
     <CODE
CLASS="FUNCTION"
>regexp_split_to_table</CODE
>, except that <CODE
CLASS="FUNCTION"
>regexp_split_to_array</CODE
>
     returns its result as an array of <TT
CLASS="TYPE"
>text</TT
>.  It has the syntax
     <CODE
CLASS="FUNCTION"
>regexp_split_to_array</CODE
>(<TT
CLASS="REPLACEABLE"
><I
>string</I
></TT
>, <TT
CLASS="REPLACEABLE"
><I
>pattern</I
></TT
>
     [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> </SPAN
>]).
     The parameters are the same as for <CODE
CLASS="FUNCTION"
>regexp_split_to_table</CODE
>.
    </P
><P
>    Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>&#13;SELECT foo FROM regexp_split_to_table('the quick brown fox jumped over the lazy dog', E'\\s+') AS foo;
  foo   
--------
 the    
 quick  
 brown  
 fox    
 jumped 
 over   
 the    
 lazy   
 dog    
(9 rows)

SELECT regexp_split_to_array('the quick brown fox jumped over the lazy dog', E'\\s+');
              regexp_split_to_array             
------------------------------------------------
 {the,quick,brown,fox,jumped,over,the,lazy,dog}
(1 row)

SELECT foo FROM regexp_split_to_table('the quick brown fox', E'\\s*') AS foo;
 foo 
-----
 t         
 h         
 e         
 q         
 u         
 i         
 c         
 k         
 b         
 r         
 o         
 w         
 n         
 f         
 o         
 x         
(16 rows)</PRE
><P>
   </P
><P
>    As the last example demonstrates, the regexp split functions ignore
    zero-length matches that occur at the start or end of the string
    or immediately after a previous match.  This is contrary to the strict
    definition of regexp matching that is implemented by
    <CODE
CLASS="FUNCTION"
>regexp_matches</CODE
>, but is usually the most convenient behavior
    in practice.  Other software systems such as Perl use similar definitions.
   </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-SYNTAX-DETAILS"
>9.7.3.1. Regular Expression Details</A
></H3
><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s regular expressions are implemented
    using a software package written by Henry Spencer.  Much of
    the description of regular expressions below is copied verbatim from his
    manual.
   </P
><P
>    Regular expressions (<ACRONYM
CLASS="ACRONYM"
>RE</ACRONYM
>s), as defined in
    <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> 1003.2, come in two forms:
    <I
CLASS="FIRSTTERM"
>extended</I
> <ACRONYM
CLASS="ACRONYM"
>RE</ACRONYM
>s or <ACRONYM
CLASS="ACRONYM"
>ERE</ACRONYM
>s
    (roughly those of <TT
CLASS="COMMAND"
>egrep</TT
>), and
    <I
CLASS="FIRSTTERM"
>basic</I
> <ACRONYM
CLASS="ACRONYM"
>RE</ACRONYM
>s or <ACRONYM
CLASS="ACRONYM"
>BRE</ACRONYM
>s
    (roughly those of <TT
CLASS="COMMAND"
>ed</TT
>).
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports both forms, and
    also implements some extensions
    that are not in the POSIX standard, but have become widely used
    due to their availability in programming languages such as Perl and Tcl.
    <ACRONYM
CLASS="ACRONYM"
>RE</ACRONYM
>s using these non-POSIX extensions are called
    <I
CLASS="FIRSTTERM"
>advanced</I
> <ACRONYM
CLASS="ACRONYM"
>RE</ACRONYM
>s or <ACRONYM
CLASS="ACRONYM"
>ARE</ACRONYM
>s
    in this documentation.  AREs are almost an exact superset of EREs,
    but BREs have several notational incompatibilities (as well as being
    much more limited).
    We first describe the ARE and ERE forms, noting features that apply
    only to AREs, and then describe how BREs differ.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> always initially presumes that a regular
     expression follows the ARE rules.  However, the more limited ERE or
     BRE rules can be chosen by prepending an <I
CLASS="FIRSTTERM"
>embedded option</I
>
     to the RE pattern, as described in <A
HREF="functions-matching.html#POSIX-METASYNTAX"
>Section 9.7.3.4</A
>.
     This can be useful for compatibility with applications that expect
     exactly the <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> 1003.2 rules.
    </P
></BLOCKQUOTE
></DIV
><P
>    A regular expression is defined as one or more
    <I
CLASS="FIRSTTERM"
>branches</I
>, separated by
    <TT
CLASS="LITERAL"
>|</TT
>.  It matches anything that matches one of the
    branches.
   </P
><P
>    A branch is zero or more <I
CLASS="FIRSTTERM"
>quantified atoms</I
> or
    <I
CLASS="FIRSTTERM"
>constraints</I
>, concatenated.
    It matches a match for the first, followed by a match for the second, etc;
    an empty branch matches the empty string.
   </P
><P
>    A quantified atom is an <I
CLASS="FIRSTTERM"
>atom</I
> possibly followed
    by a single <I
CLASS="FIRSTTERM"
>quantifier</I
>.
    Without a quantifier, it matches a match for the atom.
    With a quantifier, it can match some number of matches of the atom.
    An <I
CLASS="FIRSTTERM"
>atom</I
> can be any of the possibilities
    shown in <A
HREF="functions-matching.html#POSIX-ATOMS-TABLE"
>Table 9-12</A
>.
    The possible quantifiers and their meanings are shown in
    <A
HREF="functions-matching.html#POSIX-QUANTIFIERS-TABLE"
>Table 9-13</A
>.
   </P
><P
>    A <I
CLASS="FIRSTTERM"
>constraint</I
> matches an empty string, but matches only when
    specific conditions are met.  A constraint can be used where an atom
    could be used, except it cannot be followed by a quantifier.
    The simple constraints are shown in
    <A
HREF="functions-matching.html#POSIX-CONSTRAINTS-TABLE"
>Table 9-14</A
>;
    some more constraints are described later.
   </P
><DIV
CLASS="TABLE"
><A
NAME="POSIX-ATOMS-TABLE"
></A
><P
><B
>Table 9-12. Regular Expression Atoms</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Atom</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>(</TT
><TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
><TT
CLASS="LITERAL"
>)</TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
> is any regular expression)
       matches a match for
       <TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
>, with the match noted for possible reporting </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>(?:</TT
><TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
><TT
CLASS="LITERAL"
>)</TT
> </TD
><TD
> as above, but the match is not noted for reporting
       (a <SPAN
CLASS="QUOTE"
>"non-capturing"</SPAN
> set of parentheses)
       (AREs only) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>.</TT
> </TD
><TD
> matches any single character </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>[</TT
><TT
CLASS="REPLACEABLE"
><I
>chars</I
></TT
><TT
CLASS="LITERAL"
>]</TT
> </TD
><TD
> a <I
CLASS="FIRSTTERM"
>bracket expression</I
>,
       matching any one of the <TT
CLASS="REPLACEABLE"
><I
>chars</I
></TT
> (see
       <A
HREF="functions-matching.html#POSIX-BRACKET-EXPRESSIONS"
>Section 9.7.3.2</A
> for more detail) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>k</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>k</I
></TT
> is a non-alphanumeric character)
       matches that character taken as an ordinary character,
       e.g., <TT
CLASS="LITERAL"
>\\</TT
> matches a backslash character </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>c</I
></TT
> </TD
><TD
> where <TT
CLASS="REPLACEABLE"
><I
>c</I
></TT
> is alphanumeric
       (possibly followed by other characters)
       is an <I
CLASS="FIRSTTERM"
>escape</I
>, see <A
HREF="functions-matching.html#POSIX-ESCAPE-SEQUENCES"
>Section 9.7.3.3</A
>
       (AREs only; in EREs and BREs, this matches <TT
CLASS="REPLACEABLE"
><I
>c</I
></TT
>) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>{</TT
> </TD
><TD
> when followed by a character other than a digit,
       matches the left-brace character <TT
CLASS="LITERAL"
>{</TT
>;
       when followed by a digit, it is the beginning of a
       <TT
CLASS="REPLACEABLE"
><I
>bound</I
></TT
> (see below) </TD
></TR
><TR
><TD
> <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> </TD
><TD
> where <TT
CLASS="REPLACEABLE"
><I
>x</I
></TT
> is a single character with no other
       significance, matches that character </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    An RE cannot end with <TT
CLASS="LITERAL"
>\</TT
>.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     Remember that the backslash (<TT
CLASS="LITERAL"
>\</TT
>) already has a special
     meaning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> string literals.
     To write a pattern constant that contains a backslash,
     you must write two backslashes in the statement, assuming escape
     string syntax is used (see <A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS"
>Section 4.1.2.1</A
>).
    </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="TABLE"
><A
NAME="POSIX-QUANTIFIERS-TABLE"
></A
><P
><B
>Table 9-13. Regular Expression Quantifiers</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Quantifier</TH
><TH
>Matches</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>*</TT
> </TD
><TD
> a sequence of 0 or more matches of the atom </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>+</TT
> </TD
><TD
> a sequence of 1 or more matches of the atom </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>?</TT
> </TD
><TD
> a sequence of 0 or 1 matches of the atom </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}</TT
> </TD
><TD
> a sequence of exactly <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> matches of the atom </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,}</TT
> </TD
><TD
> a sequence of <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> or more matches of the atom </TD
></TR
><TR
><TD
>       <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}</TT
> </TD
><TD
> a sequence of <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> through <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>
       (inclusive) matches of the atom; <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> cannot exceed
       <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>*?</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>*</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>+?</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>+</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>??</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>?</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}?</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,}?</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,}</TT
> </TD
></TR
><TR
><TD
>       <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}?</TT
> </TD
><TD
> non-greedy version of <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}</TT
> </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    The forms using <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>...</I
></TT
><TT
CLASS="LITERAL"
>}</TT
>
    are known as <I
CLASS="FIRSTTERM"
>bounds</I
>.
    The numbers <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> and <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> within a bound are
    unsigned decimal integers with permissible values from 0 to 255 inclusive.
   </P
><P
>     <I
CLASS="FIRSTTERM"
>Non-greedy</I
> quantifiers (available in AREs only) match the
     same possibilities as their corresponding normal (<I
CLASS="FIRSTTERM"
>greedy</I
>)
     counterparts, but prefer the smallest number rather than the largest
     number of matches.
     See <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
> for more detail.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     A quantifier cannot immediately follow another quantifier, e.g.,
     <TT
CLASS="LITERAL"
>**</TT
> is invalid.
     A quantifier cannot
     begin an expression or subexpression or follow
     <TT
CLASS="LITERAL"
>^</TT
> or <TT
CLASS="LITERAL"
>|</TT
>.
    </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="TABLE"
><A
NAME="POSIX-CONSTRAINTS-TABLE"
></A
><P
><B
>Table 9-14. Regular Expression Constraints</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Constraint</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>^</TT
> </TD
><TD
> matches at the beginning of the string </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>$</TT
> </TD
><TD
> matches at the end of the string </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>(?=</TT
><TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
><TT
CLASS="LITERAL"
>)</TT
> </TD
><TD
> <I
CLASS="FIRSTTERM"
>positive lookahead</I
> matches at any point
       where a substring matching <TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
> begins
       (AREs only) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>(?!</TT
><TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
><TT
CLASS="LITERAL"
>)</TT
> </TD
><TD
> <I
CLASS="FIRSTTERM"
>negative lookahead</I
> matches at any point
       where no substring matching <TT
CLASS="REPLACEABLE"
><I
>re</I
></TT
> begins
       (AREs only) </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    Lookahead constraints cannot contain <I
CLASS="FIRSTTERM"
>back references</I
>
    (see <A
HREF="functions-matching.html#POSIX-ESCAPE-SEQUENCES"
>Section 9.7.3.3</A
>),
    and all parentheses within them are considered non-capturing.
   </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-BRACKET-EXPRESSIONS"
>9.7.3.2. Bracket Expressions</A
></H3
><P
>    A <I
CLASS="FIRSTTERM"
>bracket expression</I
> is a list of
    characters enclosed in <TT
CLASS="LITERAL"
>[]</TT
>.  It normally matches
    any single character from the list (but see below).  If the list
    begins with <TT
CLASS="LITERAL"
>^</TT
>, it matches any single character
    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> from the rest of the list.
    If two characters
    in the list are separated by <TT
CLASS="LITERAL"
>-</TT
>, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g., <TT
CLASS="LITERAL"
>[0-9]</TT
> in <ACRONYM
CLASS="ACRONYM"
>ASCII</ACRONYM
> matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.,  <TT
CLASS="LITERAL"
>a-c-e</TT
>.  Ranges are very
    collating-sequence-dependent, so portable programs should avoid
    relying on them.
   </P
><P
>    To include a literal <TT
CLASS="LITERAL"
>]</TT
> in the list, make it the
    first character (after <TT
CLASS="LITERAL"
>^</TT
>, if that is used).  To
    include a literal <TT
CLASS="LITERAL"
>-</TT
>, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    <TT
CLASS="LITERAL"
>-</TT
> as the first endpoint of a range, enclose it
    in <TT
CLASS="LITERAL"
>[.</TT
> and <TT
CLASS="LITERAL"
>.]</TT
> to make it a
    collating element (see below).  With the exception of these characters,
    some combinations using <TT
CLASS="LITERAL"
>[</TT
>
    (see next paragraphs), and escapes (AREs only), all other special
    characters lose their special significance within a bracket expression.
    In particular, <TT
CLASS="LITERAL"
>\</TT
> is not special when following
    ERE or BRE rules, though it is special (as introducing an escape)
    in AREs.
   </P
><P
>    Within a bracket expression, a collating element (a character, a
    multiple-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    <TT
CLASS="LITERAL"
>[.</TT
> and <TT
CLASS="LITERAL"
>.]</TT
> stands for the
    sequence of characters of that collating element.  The sequence is
    treated as a single element of the bracket expression's list.  This
    allows a bracket
    expression containing a multiple-character collating element to
    match more than one character, e.g., if the collating sequence
    includes a <TT
CLASS="LITERAL"
>ch</TT
> collating element, then the RE
    <TT
CLASS="LITERAL"
>[[.ch.]]*c</TT
> matches the first five characters of
    <TT
CLASS="LITERAL"
>chchcc</TT
>.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> currently does not support multi-character collating
     elements. This information describes possible future behavior.
    </P
></BLOCKQUOTE
></DIV
><P
>    Within a bracket expression, a collating element enclosed in
    <TT
CLASS="LITERAL"
>[=</TT
> and <TT
CLASS="LITERAL"
>=]</TT
> is an <I
CLASS="FIRSTTERM"
>equivalence
    class</I
>, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were <TT
CLASS="LITERAL"
>[.</TT
> and
    <TT
CLASS="LITERAL"
>.]</TT
>.)  For example, if <TT
CLASS="LITERAL"
>o</TT
> and
    <TT
CLASS="LITERAL"
>^</TT
> are the members of an equivalence class, then
    <TT
CLASS="LITERAL"
>[[=o=]]</TT
>, <TT
CLASS="LITERAL"
>[[=^=]]</TT
>, and
    <TT
CLASS="LITERAL"
>[o^]</TT
> are all synonymous.  An equivalence class
    cannot be an endpoint of a range.
   </P
><P
>    Within a bracket expression, the name of a character class
    enclosed in <TT
CLASS="LITERAL"
>[:</TT
> and <TT
CLASS="LITERAL"
>:]</TT
> stands
    for the list of all characters belonging to that class.  Standard
    character class names are: <TT
CLASS="LITERAL"
>alnum</TT
>,
    <TT
CLASS="LITERAL"
>alpha</TT
>, <TT
CLASS="LITERAL"
>blank</TT
>,
    <TT
CLASS="LITERAL"
>cntrl</TT
>, <TT
CLASS="LITERAL"
>digit</TT
>,
    <TT
CLASS="LITERAL"
>graph</TT
>, <TT
CLASS="LITERAL"
>lower</TT
>,
    <TT
CLASS="LITERAL"
>print</TT
>, <TT
CLASS="LITERAL"
>punct</TT
>,
    <TT
CLASS="LITERAL"
>space</TT
>, <TT
CLASS="LITERAL"
>upper</TT
>,
    <TT
CLASS="LITERAL"
>xdigit</TT
>.  These stand for the character classes
    defined in
    <SPAN
CLASS="CITEREFENTRY"
><SPAN
CLASS="REFENTRYTITLE"
>ctype</SPAN
></SPAN
>.
    A locale can provide others.  A character class cannot be used as
    an endpoint of a range.
   </P
><P
>    There are two special cases of bracket expressions:  the bracket
    expressions <TT
CLASS="LITERAL"
>[[:&lt;:]]</TT
> and
    <TT
CLASS="LITERAL"
>[[:&gt;:]]</TT
> are constraints,
    matching empty strings at the beginning
    and end of a word respectively.  A word is defined as a sequence
    of word characters that is neither preceded nor followed by word
    characters.  A word character is an <TT
CLASS="LITERAL"
>alnum</TT
> character (as
    defined by
    <SPAN
CLASS="CITEREFENTRY"
><SPAN
CLASS="REFENTRYTITLE"
>ctype</SPAN
></SPAN
>)
    or an underscore.  This is an extension, compatible with but not
    specified by <ACRONYM
CLASS="ACRONYM"
>POSIX</ACRONYM
> 1003.2, and should be used with
    caution in software intended to be portable to other systems.
    The constraint escapes described below are usually preferable; they
    are no more standard, but are easier to type.
   </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-ESCAPE-SEQUENCES"
>9.7.3.3. Regular Expression Escapes</A
></H3
><P
>    <I
CLASS="FIRSTTERM"
>Escapes</I
> are special sequences beginning with <TT
CLASS="LITERAL"
>\</TT
>
    followed by an alphanumeric character. Escapes come in several varieties:
    character entry, class shorthands, constraint escapes, and back references.
    A <TT
CLASS="LITERAL"
>\</TT
> followed by an alphanumeric character but not constituting
    a valid escape is illegal in AREs.
    In EREs, there are no escapes: outside a bracket expression,
    a <TT
CLASS="LITERAL"
>\</TT
> followed by an alphanumeric character merely stands for
    that character as an ordinary character, and inside a bracket expression,
    <TT
CLASS="LITERAL"
>\</TT
> is an ordinary character.
    (The latter is the one actual incompatibility between EREs and AREs.)
   </P
><P
>    <I
CLASS="FIRSTTERM"
>Character-entry escapes</I
> exist to make it easier to specify
    non-printing and other inconvenient characters in REs.  They are
    shown in <A
HREF="functions-matching.html#POSIX-CHARACTER-ENTRY-ESCAPES-TABLE"
>Table 9-15</A
>.
   </P
><P
>    <I
CLASS="FIRSTTERM"
>Class-shorthand escapes</I
> provide shorthands for certain
    commonly-used character classes.  They are
    shown in <A
HREF="functions-matching.html#POSIX-CLASS-SHORTHAND-ESCAPES-TABLE"
>Table 9-16</A
>.
   </P
><P
>    A <I
CLASS="FIRSTTERM"
>constraint escape</I
> is a constraint,
    matching the empty string if specific conditions are met,
    written as an escape.  They are
    shown in <A
HREF="functions-matching.html#POSIX-CONSTRAINT-ESCAPES-TABLE"
>Table 9-17</A
>.
   </P
><P
>    A <I
CLASS="FIRSTTERM"
>back reference</I
> (<TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>) matches the
    same string matched by the previous parenthesized subexpression specified
    by the number <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>
    (see <A
HREF="functions-matching.html#POSIX-CONSTRAINT-BACKREF-TABLE"
>Table 9-18</A
>).  For example,
    <TT
CLASS="LITERAL"
>([bc])\1</TT
> matches <TT
CLASS="LITERAL"
>bb</TT
> or <TT
CLASS="LITERAL"
>cc</TT
>
    but not <TT
CLASS="LITERAL"
>bc</TT
> or <TT
CLASS="LITERAL"
>cb</TT
>.
    The subexpression must entirely precede the back reference in the RE.
    Subexpressions are numbered in the order of their leading parentheses.
    Non-capturing parentheses do not define subexpressions.
   </P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     Keep in mind that an escape's leading <TT
CLASS="LITERAL"
>\</TT
> will need to be
     doubled when entering the pattern as an SQL string constant.  For example:
</P><PRE
CLASS="PROGRAMLISTING"
>'123' ~ E'^\\d{3}' <I
CLASS="LINEANNOTATION"
>true</I
></PRE
><P>
    </P
></BLOCKQUOTE
></DIV
><DIV
CLASS="TABLE"
><A
NAME="POSIX-CHARACTER-ENTRY-ESCAPES-TABLE"
></A
><P
><B
>Table 9-15. Regular Expression Character-entry Escapes</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Escape</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>\a</TT
> </TD
><TD
> alert (bell) character, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\b</TT
> </TD
><TD
> backspace, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\B</TT
> </TD
><TD
> synonym for backslash (<TT
CLASS="LITERAL"
>\</TT
>) to help reduce the need for backslash
       doubling </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\c</TT
><TT
CLASS="REPLACEABLE"
><I
>X</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>X</I
></TT
> is any character) the character whose
       low-order 5 bits are the same as those of
       <TT
CLASS="REPLACEABLE"
><I
>X</I
></TT
>, and whose other bits are all zero </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\e</TT
> </TD
><TD
> the character whose collating-sequence name
       is <TT
CLASS="LITERAL"
>ESC</TT
>,
       or failing that, the character with octal value 033 </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\f</TT
> </TD
><TD
> form feed, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\n</TT
> </TD
><TD
> newline, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\r</TT
> </TD
><TD
> carriage return, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\t</TT
> </TD
><TD
> horizontal tab, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\u</TT
><TT
CLASS="REPLACEABLE"
><I
>wxyz</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>wxyz</I
></TT
> is exactly four hexadecimal digits)
       the UTF16 (Unicode, 16-bit) character <TT
CLASS="LITERAL"
>U+</TT
><TT
CLASS="REPLACEABLE"
><I
>wxyz</I
></TT
>
       in the local byte ordering </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\U</TT
><TT
CLASS="REPLACEABLE"
><I
>stuvwxyz</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>stuvwxyz</I
></TT
> is exactly eight hexadecimal
       digits)
       reserved for a hypothetical Unicode extension to 32 bits
       </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\v</TT
> </TD
><TD
> vertical tab, as in C </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\x</TT
><TT
CLASS="REPLACEABLE"
><I
>hhh</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>hhh</I
></TT
> is any sequence of hexadecimal
       digits)
       the character whose hexadecimal value is
       <TT
CLASS="LITERAL"
>0x</TT
><TT
CLASS="REPLACEABLE"
><I
>hhh</I
></TT
>
       (a single character no matter how many hexadecimal digits are used)
       </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\0</TT
> </TD
><TD
> the character whose value is <TT
CLASS="LITERAL"
>0</TT
> (the null byte)</TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>xy</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>xy</I
></TT
> is exactly two octal digits,
       and is not a <I
CLASS="FIRSTTERM"
>back reference</I
>)
       the character whose octal value is
       <TT
CLASS="LITERAL"
>0</TT
><TT
CLASS="REPLACEABLE"
><I
>xy</I
></TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>xyz</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>xyz</I
></TT
> is exactly three octal digits,
       and is not a <I
CLASS="FIRSTTERM"
>back reference</I
>)
       the character whose octal value is
       <TT
CLASS="LITERAL"
>0</TT
><TT
CLASS="REPLACEABLE"
><I
>xyz</I
></TT
> </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    Hexadecimal digits are <TT
CLASS="LITERAL"
>0</TT
>-<TT
CLASS="LITERAL"
>9</TT
>,
    <TT
CLASS="LITERAL"
>a</TT
>-<TT
CLASS="LITERAL"
>f</TT
>, and <TT
CLASS="LITERAL"
>A</TT
>-<TT
CLASS="LITERAL"
>F</TT
>.
    Octal digits are <TT
CLASS="LITERAL"
>0</TT
>-<TT
CLASS="LITERAL"
>7</TT
>.
   </P
><P
>    The character-entry escapes are always taken as ordinary characters.
    For example, <TT
CLASS="LITERAL"
>\135</TT
> is <TT
CLASS="LITERAL"
>]</TT
> in ASCII, but
    <TT
CLASS="LITERAL"
>\135</TT
> does not terminate a bracket expression.
   </P
><DIV
CLASS="TABLE"
><A
NAME="POSIX-CLASS-SHORTHAND-ESCAPES-TABLE"
></A
><P
><B
>Table 9-16. Regular Expression Class-shorthand Escapes</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Escape</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>\d</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[[:digit:]]</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\s</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[[:space:]]</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\w</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[[:alnum:]_]</TT
>
       (note underscore is included) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\D</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[^[:digit:]]</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\S</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[^[:space:]]</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\W</TT
> </TD
><TD
> <TT
CLASS="LITERAL"
>[^[:alnum:]_]</TT
>
       (note underscore is included) </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    Within bracket expressions, <TT
CLASS="LITERAL"
>\d</TT
>, <TT
CLASS="LITERAL"
>\s</TT
>,
    and <TT
CLASS="LITERAL"
>\w</TT
> lose their outer brackets,
    and <TT
CLASS="LITERAL"
>\D</TT
>, <TT
CLASS="LITERAL"
>\S</TT
>, and <TT
CLASS="LITERAL"
>\W</TT
> are illegal.
    (So, for example, <TT
CLASS="LITERAL"
>[a-c\d]</TT
> is equivalent to
    <TT
CLASS="LITERAL"
>[a-c[:digit:]]</TT
>.
    Also, <TT
CLASS="LITERAL"
>[a-c\D]</TT
>, which is equivalent to
    <TT
CLASS="LITERAL"
>[a-c^[:digit:]]</TT
>, is illegal.)
   </P
><DIV
CLASS="TABLE"
><A
NAME="POSIX-CONSTRAINT-ESCAPES-TABLE"
></A
><P
><B
>Table 9-17. Regular Expression Constraint Escapes</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Escape</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>\A</TT
> </TD
><TD
> matches only at the beginning of the string
       (see <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
> for how this differs from
       <TT
CLASS="LITERAL"
>^</TT
>) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\m</TT
> </TD
><TD
> matches only at the beginning of a word </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\M</TT
> </TD
><TD
> matches only at the end of a word </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\y</TT
> </TD
><TD
> matches only at the beginning or end of a word </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\Y</TT
> </TD
><TD
> matches only at a point that is not the beginning or end of a
       word </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\Z</TT
> </TD
><TD
> matches only at the end of the string
       (see <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
> for how this differs from
       <TT
CLASS="LITERAL"
>$</TT
>) </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    A word is defined as in the specification of
    <TT
CLASS="LITERAL"
>[[:&lt;:]]</TT
> and <TT
CLASS="LITERAL"
>[[:&gt;:]]</TT
> above.
    Constraint escapes are illegal within bracket expressions.
   </P
><DIV
CLASS="TABLE"
><A
NAME="POSIX-CONSTRAINT-BACKREF-TABLE"
></A
><P
><B
>Table 9-18. Regular Expression Back References</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Escape</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> is a nonzero digit)
       a back reference to the <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
>'th subexpression </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>\</TT
><TT
CLASS="REPLACEABLE"
><I
>mnn</I
></TT
> </TD
><TD
> (where <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> is a nonzero digit, and
       <TT
CLASS="REPLACEABLE"
><I
>nn</I
></TT
> is some more digits, and the decimal value
       <TT
CLASS="REPLACEABLE"
><I
>mnn</I
></TT
> is not greater than the number of closing capturing
       parentheses seen so far)
       a back reference to the <TT
CLASS="REPLACEABLE"
><I
>mnn</I
></TT
>'th subexpression </TD
></TR
></TBODY
></TABLE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     There is an inherent ambiguity between octal character-entry
     escapes and back references, which is resolved by the following heuristics,
     as hinted at above.
     A leading zero always indicates an octal escape.
     A single non-zero digit, not followed by another digit,
     is always taken as a back reference.
     A multi-digit sequence not starting with a zero is taken as a back
     reference if it comes after a suitable subexpression
     (i.e., the number is in the legal range for a back reference),
     and otherwise is taken as octal.
    </P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-METASYNTAX"
>9.7.3.4. Regular Expression Metasyntax</A
></H3
><P
>    In addition to the main syntax described above, there are some special
    forms and miscellaneous syntactic facilities available.
   </P
><P
>    An RE can begin with one of two special <I
CLASS="FIRSTTERM"
>director</I
> prefixes.
    If an RE begins with <TT
CLASS="LITERAL"
>***:</TT
>,
    the rest of the RE is taken as an ARE.  (This normally has no effect in
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, since REs are assumed to be AREs;
    but it does have an effect if ERE or BRE mode had been specified by
    the <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> parameter to a regex function.)
    If an RE begins with <TT
CLASS="LITERAL"
>***=</TT
>,
    the rest of the RE is taken to be a literal string,
    with all characters considered ordinary characters.
   </P
><P
>    An ARE can begin with <I
CLASS="FIRSTTERM"
>embedded options</I
>:
    a sequence <TT
CLASS="LITERAL"
>(?</TT
><TT
CLASS="REPLACEABLE"
><I
>xyz</I
></TT
><TT
CLASS="LITERAL"
>)</TT
>
    (where <TT
CLASS="REPLACEABLE"
><I
>xyz</I
></TT
> is one or more alphabetic characters)
    specifies options affecting the rest of the RE.
    These options override any previously determined options &mdash;
    in particular, they can override the case-sensitivity behavior implied by
    a regex operator, or the <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
> parameter to a regex
    function.
    The available option letters are
    shown in <A
HREF="functions-matching.html#POSIX-EMBEDDED-OPTIONS-TABLE"
>Table 9-19</A
>.
    Note that these same option letters are used in the <TT
CLASS="REPLACEABLE"
><I
>flags</I
></TT
>
    parameters of regex functions.
   </P
><DIV
CLASS="TABLE"
><A
NAME="POSIX-EMBEDDED-OPTIONS-TABLE"
></A
><P
><B
>Table 9-19. ARE Embedded-option Letters</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><THEAD
><TR
><TH
>Option</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
> <TT
CLASS="LITERAL"
>b</TT
> </TD
><TD
> rest of RE is a BRE </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>c</TT
> </TD
><TD
> case-sensitive matching (overrides operator type) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>e</TT
> </TD
><TD
> rest of RE is an ERE </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>i</TT
> </TD
><TD
> case-insensitive matching (see
       <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
>) (overrides operator type) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>m</TT
> </TD
><TD
> historical synonym for <TT
CLASS="LITERAL"
>n</TT
> </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>n</TT
> </TD
><TD
> newline-sensitive matching (see
       <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
>) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>p</TT
> </TD
><TD
> partial newline-sensitive matching (see
       <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
>) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>q</TT
> </TD
><TD
> rest of RE is a literal (<SPAN
CLASS="QUOTE"
>"quoted"</SPAN
>) string, all ordinary
       characters </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>s</TT
> </TD
><TD
> non-newline-sensitive matching (default) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>t</TT
> </TD
><TD
> tight syntax (default; see below) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>w</TT
> </TD
><TD
> inverse partial newline-sensitive (<SPAN
CLASS="QUOTE"
>"weird"</SPAN
>) matching
       (see <A
HREF="functions-matching.html#POSIX-MATCHING-RULES"
>Section 9.7.3.5</A
>) </TD
></TR
><TR
><TD
> <TT
CLASS="LITERAL"
>x</TT
> </TD
><TD
> expanded syntax (see below) </TD
></TR
></TBODY
></TABLE
></DIV
><P
>    Embedded options take effect at the <TT
CLASS="LITERAL"
>)</TT
> terminating the sequence.
    They can appear only at the start of an ARE (after the
    <TT
CLASS="LITERAL"
>***:</TT
> director if any).
   </P
><P
>    In addition to the usual (<I
CLASS="FIRSTTERM"
>tight</I
>) RE syntax, in which all
    characters are significant, there is an <I
CLASS="FIRSTTERM"
>expanded</I
> syntax,
    available by specifying the embedded <TT
CLASS="LITERAL"
>x</TT
> option.
    In the expanded syntax,
    white-space characters in the RE are ignored, as are
    all characters between a <TT
CLASS="LITERAL"
>#</TT
>
    and the following newline (or the end of the RE).  This
    permits paragraphing and commenting a complex RE.
    There are three exceptions to that basic rule:

    <P
></P
></P><UL
><LI
><P
>       a white-space character or <TT
CLASS="LITERAL"
>#</TT
> preceded by <TT
CLASS="LITERAL"
>\</TT
> is
       retained
      </P
></LI
><LI
><P
>       white space or <TT
CLASS="LITERAL"
>#</TT
> within a bracket expression is retained
      </P
></LI
><LI
><P
>       white space and comments cannot appear within multi-character symbols,
       such as <TT
CLASS="LITERAL"
>(?:</TT
>
      </P
></LI
></UL
><P>

    For this purpose, white-space characters are blank, tab, newline, and
    any character that belongs to the <TT
CLASS="REPLACEABLE"
><I
>space</I
></TT
> character class.
   </P
><P
>    Finally, in an ARE, outside bracket expressions, the sequence
    <TT
CLASS="LITERAL"
>(?#</TT
><TT
CLASS="REPLACEABLE"
><I
>ttt</I
></TT
><TT
CLASS="LITERAL"
>)</TT
>
    (where <TT
CLASS="REPLACEABLE"
><I
>ttt</I
></TT
> is any text not containing a <TT
CLASS="LITERAL"
>)</TT
>)
    is a comment, completely ignored.
    Again, this is not allowed between the characters of
    multi-character symbols, like <TT
CLASS="LITERAL"
>(?:</TT
>.
    Such comments are more a historical artifact than a useful facility,
    and their use is deprecated; use the expanded syntax instead.
   </P
><P
>    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>None</I
></SPAN
> of these metasyntax extensions is available if
    an initial <TT
CLASS="LITERAL"
>***=</TT
> director
    has specified that the user's input be treated as a literal string
    rather than as an RE.
   </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-MATCHING-RULES"
>9.7.3.5. Regular Expression Matching Rules</A
></H3
><P
>    In the event that an RE could match more than one substring of a given
    string, the RE matches the one starting earliest in the string.
    If the RE could match more than one substring starting at that point,
    either the longest possible match or the shortest possible match will
    be taken, depending on whether the RE is <I
CLASS="FIRSTTERM"
>greedy</I
> or
    <I
CLASS="FIRSTTERM"
>non-greedy</I
>.
   </P
><P
>    Whether an RE is greedy or not is determined by the following rules:
    <P
></P
></P><UL
><LI
><P
>       Most atoms, and all constraints, have no greediness attribute (because
       they cannot match variable amounts of text anyway).
      </P
></LI
><LI
><P
>       Adding parentheses around an RE does not change its greediness.
      </P
></LI
><LI
><P
>       A quantified atom with a fixed-repetition quantifier
       (<TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}</TT
>
       or
       <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>}?</TT
>)
       has the same greediness (possibly none) as the atom itself.
      </P
></LI
><LI
><P
>       A quantified atom with other normal quantifiers (including
       <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}</TT
>
       with <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> equal to <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)
       is greedy (prefers longest match).
      </P
></LI
><LI
><P
>       A quantified atom with a non-greedy quantifier (including
       <TT
CLASS="LITERAL"
>{</TT
><TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
><TT
CLASS="LITERAL"
>,</TT
><TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
><TT
CLASS="LITERAL"
>}?</TT
>
       with <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> equal to <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>)
       is non-greedy (prefers shortest match).
      </P
></LI
><LI
><P
>       A branch &mdash; that is, an RE that has no top-level
       <TT
CLASS="LITERAL"
>|</TT
> operator &mdash; has the same greediness as the first
       quantified atom in it that has a greediness attribute.
      </P
></LI
><LI
><P
>       An RE consisting of two or more branches connected by the
       <TT
CLASS="LITERAL"
>|</TT
> operator is always greedy.
      </P
></LI
></UL
><P>
   </P
><P
>    The above rules associate greediness attributes not only with individual
    quantified atoms, but with branches and entire REs that contain quantified
    atoms.  What that means is that the matching is done in such a way that
    the branch, or whole RE, matches the longest or shortest possible
    substring <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>as a whole</I
></SPAN
>.  Once the length of the entire match
    is determined, the part of it that matches any particular subexpression
    is determined on the basis of the greediness attribute of that
    subexpression, with subexpressions starting earlier in the RE taking
    priority over ones starting later.
   </P
><P
>    An example of what this means:
</P><PRE
CLASS="SCREEN"
>SELECT SUBSTRING('XY1234Z', 'Y*([0-9]{1,3})');
<I
CLASS="LINEANNOTATION"
>Result: </I
><SAMP
CLASS="COMPUTEROUTPUT"
>123</SAMP
>
SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
<I
CLASS="LINEANNOTATION"
>Result: </I
><SAMP
CLASS="COMPUTEROUTPUT"
>1</SAMP
></PRE
><P>
    In the first case, the RE as a whole is greedy because <TT
CLASS="LITERAL"
>Y*</TT
>
    is greedy.  It can match beginning at the <TT
CLASS="LITERAL"
>Y</TT
>, and it matches
    the longest possible string starting there, i.e., <TT
CLASS="LITERAL"
>Y123</TT
>.
    The output is the parenthesized part of that, or <TT
CLASS="LITERAL"
>123</TT
>.
    In the second case, the RE as a whole is non-greedy because <TT
CLASS="LITERAL"
>Y*?</TT
>
    is non-greedy.  It can match beginning at the <TT
CLASS="LITERAL"
>Y</TT
>, and it matches
    the shortest possible string starting there, i.e., <TT
CLASS="LITERAL"
>Y1</TT
>.
    The subexpression <TT
CLASS="LITERAL"
>[0-9]{1,3}</TT
> is greedy but it cannot change
    the decision as to the overall match length; so it is forced to match
    just <TT
CLASS="LITERAL"
>1</TT
>.
   </P
><P
>    In short, when an RE contains both greedy and non-greedy subexpressions,
    the total match length is either as long as possible or as short as
    possible, according to the attribute assigned to the whole RE.  The
    attributes assigned to the subexpressions only affect how much of that
    match they are allowed to <SPAN
CLASS="QUOTE"
>"eat"</SPAN
> relative to each other.
   </P
><P
>    The quantifiers <TT
CLASS="LITERAL"
>{1,1}</TT
> and <TT
CLASS="LITERAL"
>{1,1}?</TT
>
    can be used to force greediness or non-greediness, respectively,
    on a subexpression or a whole RE.
   </P
><P
>    Match lengths are measured in characters, not collating elements.
    An empty string is considered longer than no match at all.
    For example:
    <TT
CLASS="LITERAL"
>bb*</TT
>
    matches the three middle characters of <TT
CLASS="LITERAL"
>abbbc</TT
>;
    <TT
CLASS="LITERAL"
>(week|wee)(night|knights)</TT
>
    matches all ten characters of <TT
CLASS="LITERAL"
>weeknights</TT
>;
    when <TT
CLASS="LITERAL"
>(.*).*</TT
>
    is matched against <TT
CLASS="LITERAL"
>abc</TT
> the parenthesized subexpression
    matches all three characters; and when
    <TT
CLASS="LITERAL"
>(a*)*</TT
> is matched against <TT
CLASS="LITERAL"
>bc</TT
>
    both the whole RE and the parenthesized
    subexpression match an empty string.
   </P
><P
>    If case-independent matching is specified,
    the effect is much as if all case distinctions had vanished from the
    alphabet.
    When an alphabetic that exists in multiple cases appears as an
    ordinary character outside a bracket expression, it is effectively
    transformed into a bracket expression containing both cases,
    e.g., <TT
CLASS="LITERAL"
>x</TT
> becomes <TT
CLASS="LITERAL"
>[xX]</TT
>.
    When it appears inside a bracket expression, all case counterparts
    of it are added to the bracket expression, e.g.,
    <TT
CLASS="LITERAL"
>[x]</TT
> becomes <TT
CLASS="LITERAL"
>[xX]</TT
>
    and <TT
CLASS="LITERAL"
>[^x]</TT
> becomes <TT
CLASS="LITERAL"
>[^xX]</TT
>.
   </P
><P
>    If newline-sensitive matching is specified, <TT
CLASS="LITERAL"
>.</TT
>
    and bracket expressions using <TT
CLASS="LITERAL"
>^</TT
>
    will never match the newline character
    (so that matches will never cross newlines unless the RE
    explicitly arranges it)
    and <TT
CLASS="LITERAL"
>^</TT
>and <TT
CLASS="LITERAL"
>$</TT
>
    will match the empty string after and before a newline
    respectively, in addition to matching at beginning and end of string
    respectively.
    But the ARE escapes <TT
CLASS="LITERAL"
>\A</TT
> and <TT
CLASS="LITERAL"
>\Z</TT
>
    continue to match beginning or end of string <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>only</I
></SPAN
>.
   </P
><P
>    If partial newline-sensitive matching is specified,
    this affects <TT
CLASS="LITERAL"
>.</TT
> and bracket expressions
    as with newline-sensitive matching, but not <TT
CLASS="LITERAL"
>^</TT
>
    and <TT
CLASS="LITERAL"
>$</TT
>.
   </P
><P
>    If inverse partial newline-sensitive matching is specified,
    this affects <TT
CLASS="LITERAL"
>^</TT
> and <TT
CLASS="LITERAL"
>$</TT
>
    as with newline-sensitive matching, but not <TT
CLASS="LITERAL"
>.</TT
>
    and bracket expressions.
    This isn't very useful but is provided for symmetry.
   </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-LIMITS-COMPATIBILITY"
>9.7.3.6. Limits and Compatibility</A
></H3
><P
>    No particular limit is imposed on the length of REs in this
    implementation.  However,
    programs intended to be highly portable should not employ REs longer
    than 256 bytes,
    as a POSIX-compliant implementation can refuse to accept such REs.
   </P
><P
>    The only feature of AREs that is actually incompatible with
    POSIX EREs is that <TT
CLASS="LITERAL"
>\</TT
> does not lose its special
    significance inside bracket expressions.
    All other ARE features use syntax which is illegal or has
    undefined or unspecified effects in POSIX EREs;
    the <TT
CLASS="LITERAL"
>***</TT
> syntax of directors likewise is outside the POSIX
    syntax for both BREs and EREs.
   </P
><P
>    Many of the ARE extensions are borrowed from Perl, but some have
    been changed to clean them up, and a few Perl extensions are not present.
    Incompatibilities of note include <TT
CLASS="LITERAL"
>\b</TT
>, <TT
CLASS="LITERAL"
>\B</TT
>,
    the lack of special treatment for a trailing newline,
    the addition of complemented bracket expressions to the things
    affected by newline-sensitive matching,
    the restrictions on parentheses and back references in lookahead
    constraints, and the longest/shortest-match (rather than first-match)
    matching semantics.
   </P
><P
>    Two significant incompatibilities exist between AREs and the ERE syntax
    recognized by pre-7.4 releases of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>:

    <P
></P
></P><UL
><LI
><P
>       In AREs, <TT
CLASS="LITERAL"
>\</TT
> followed by an alphanumeric character is either
       an escape or an error, while in previous releases, it was just another
       way of writing the alphanumeric.
       This should not be much of a problem because there was no reason to
       write such a sequence in earlier releases.
      </P
></LI
><LI
><P
>       In AREs, <TT
CLASS="LITERAL"
>\</TT
> remains a special character within
       <TT
CLASS="LITERAL"
>[]</TT
>, so a literal <TT
CLASS="LITERAL"
>\</TT
> within a bracket
       expression must be written <TT
CLASS="LITERAL"
>\\</TT
>.
      </P
></LI
></UL
><P>
   </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="POSIX-BASIC-REGEXES"
>9.7.3.7. Basic Regular Expressions</A
></H3
><P
>    BREs differ from EREs in several respects.
    In BREs, <TT
CLASS="LITERAL"
>|</TT
>, <TT
CLASS="LITERAL"
>+</TT
>, and <TT
CLASS="LITERAL"
>?</TT
>
    are ordinary characters and there is no equivalent
    for their functionality.
    The delimiters for bounds are
    <TT
CLASS="LITERAL"
>\{</TT
> and <TT
CLASS="LITERAL"
>\}</TT
>,
    with <TT
CLASS="LITERAL"
>{</TT
> and <TT
CLASS="LITERAL"
>}</TT
>
    by themselves ordinary characters.
    The parentheses for nested subexpressions are
    <TT
CLASS="LITERAL"
>\(</TT
> and <TT
CLASS="LITERAL"
>\)</TT
>,
    with <TT
CLASS="LITERAL"
>(</TT
> and <TT
CLASS="LITERAL"
>)</TT
> by themselves ordinary characters.
    <TT
CLASS="LITERAL"
>^</TT
> is an ordinary character except at the beginning of the
    RE or the beginning of a parenthesized subexpression,
    <TT
CLASS="LITERAL"
>$</TT
> is an ordinary character except at the end of the
    RE or the end of a parenthesized subexpression,
    and <TT
CLASS="LITERAL"
>*</TT
> is an ordinary character if it appears at the beginning
    of the RE or the beginning of a parenthesized subexpression
    (after a possible leading <TT
CLASS="LITERAL"
>^</TT
>).
    Finally, single-digit back references are available, and
    <TT
CLASS="LITERAL"
>\&lt;</TT
> and <TT
CLASS="LITERAL"
>\&gt;</TT
>
    are synonyms for
    <TT
CLASS="LITERAL"
>[[:&lt;:]]</TT
> and <TT
CLASS="LITERAL"
>[[:&gt;:]]</TT
>
    respectively; no other escapes are available in BREs.
   </P
></DIV
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